The invention relates to an air-conditioning unit, especially for the passenger compartment of a vehicle, and to a method of controlling a refrigerant loop in such a unit, said loop containing a compressor suitable for receiving the refrigerant in the gaseous state and for compressing it, a refrigerant cooler suitable for cooling the refrigerant compressed by the compressor, at approximately constant pressure, by transferring heat to a first medium, an expander suitable for lowering the pressure of the refrigerant leaving the refrigerant cooler by taking it at least partly into the liquid state and an evaporator suitable for making the refrigerant in the liquid state coming from the expander pass into the gaseous state, at approximately constant pressure, by removing heat from a second medium in order to cool the space to be air-conditioned, the refrigerant thus vaporized then being sucked up by the compressor, the loop furthermore containing an internal heat exchanger allowing the refrigerant, flowing in a first path of the internal exchanger, between the refrigerant cooler and the expander, to yield heat to the refrigerant flowing in a second path of the internal exchanger, between the evaporator and the compressor.
To avoid the deleterious effects on the environment of the fluorocompounds conventionally used as refrigerants in the air-conditioning of motor vehicles, the use of carbon dioxide CO2 is recommended.
This compound has a relatively low critical pressure, which is exceeded during compression of the refrigerant by the compressor so that the refrigerant is then cooled, without any phase change, by the refrigerant cooler which replaces the condenser of the conventional loop. In the absence of a phase change, only the reduction in the temperature of the refrigerant in the cooler allows thermal energy to be dissipated. Since this dissipation generally takes place in a stream of atmospheric air, it is necessary for the temperature of the refrigerant entering the cooler to be substantially greater than the atmospheric temperature. This is why the internal heat exchanger is used, which allows the refrigerant to be warmed up when it flows between the evaporator and the cooler and allows it to be cooled when it flows between the cooler and the expander.
The efficiency xcex7 of the internal heat exchanger, represented by equation [1]:                     η        =                                            T              pi                        -                          T              eo                                                          T              co                        -                          T              eo                                                          [        1        ]            
in which Tpi, Teo and Tco are the compressor inlet temperature, the evaporator outlet temperature and the cooler outlet temperature respectively, is a decreasing function of the flow rate of refrigerant passing through it, according to equation [2]
xcex7=axc2x7Qbxe2x80x83xe2x80x83[2], 
a and b being characteristic constants of the internal exchanger.
The foregoing is true only when the internal heat exchanger receives from the evaporator refrigerant entirely in the gaseous state. If, on the contrary, it receives therefrom refrigerant in the liquid state, its efficiency is greatly reduced.
The object of the invention is to optimize the operation of the loop so as to avoid this drawback.
Moreover, in order for the stream of air cooled by the evaporator to be at a uniform temperature, it is necessary for the evaporator not to have an overheating zone, in other words for the refrigerant to vaporize right to the end of its path through the evaporator.
Another object of the invention is to satisfy this condition.
The aim of the invention is especially a method of the kind defined in the introduction and ensures that a first condition likely to reveal the presence of refrigerant in the liquid state in said second path is monitored and that the flow rate of refrigerant in the loop is reduced when said first condition is satisfied.
This mode of regulation, based on a thermodynamic principle, allows the operating conditions of the loop to be rapidly stabilized, without any oscillation. In particular, it avoids the appearance of a cold peak when the vehicle accelerates.
Optional features of the invention, which are complementary or alternative, are given below.
Said first condition consists in that the efficiency xcex7 of the internal heat exchanger, represented by equation                     η        =                                            T              pi                        -                          T              eo                                                          T              co                        -                          T              eo                                                          [        1        ]            
in which Tpi, Teo and Tco are the compressor inlet temperature, the evaporator outlet temperature and the cooler outlet temperature respectively, is less than a reference value xcex70.
A second condition likely to reveal the existence of an overheating zone in the evaporator is additionally monitored and the flow rate of the refrigerant in the loop is increased when said second condition is satisfied.
Said second condition consists in that the efficiency xcex7 as defined in claim 2 is greater than or equal to a reference value xcex70.
The flow rate of the refrigerant is set approximately to the maximum value compatible with an efficiency xcex7 not less than the reference value.
Whatever the value of the flow rate, the value xcex7m, taken by the efficiency xcex7 when the flow rate is a maximum and when said second path does not contain refrigerant in the liquid state, is adopted as reference value.
For a given value Qp of the flow rate, the value xcex7p, taken by the efficiency xcex7 when said second path does not contain refrigerant in the liquid state, is adopted as reference value.
The flow rate is set by acting upon the expander.
To evaluate xcex7 on the basis of equation [1], a value measured by means of a sensor in thermal contact with the refrigerant is used for at least one of said temperatures.
To evaluate xcex7 on the basis of equation [1], the temperature of a stream of air having swept the evaporator and constituting said second medium is used to represent Teo.
Tpi is compared with a setpoint value Tpi,set such that       η    0    =                    T                  pi          ,          set                    -              T        eo                            T        co            -              T        eo            
and xcex7 is considered to be less than and greater than the reference value when Tpi is less than and greater than said setpoint value, respectively.
The compressor is of the type with a variable swept volume with external control.
The compressor compresses the refrigerant up to a supercritical pressure.
The subject of the invention is also an air-conditioning unit, especially for the passenger compartment of a vehicle, suitable for implementing the method as defined above, comprising a refrigerant loop as defined, monitoring means for monitoring a first condition likely to reveal the presence of refrigerant in the liquid state in said second path and, optionally, a second condition likely to reveal the existence of an overheating zone in the evaporator, and means for controlling the flow rate of the refrigerant in the loop according to the result of this monitoring.
The unit according to the invention may include at least some of the following features:
the monitoring means comprise means for evaluating the temperatures Tpi, Teo and Tco at the compressor inlet, at the evaporator outlet and at the cooler outlet respectively, means for calculating from the latter the efficiency xcex7 of the internal heat exchanger on the basis of equation [1]:                     η        =                                            T              pi                        -                          T              eo                                                          T              co                        -                          T              eo                                                          [        1        ]            
and means for comparing the efficiency xcex7 with a reference value;
means for determining the flow rate of the refrigerant in the loop and for determining said reference value from the flow rate;
the means for evaluating said temperatures comprise at least one temperature sensor in thermal contact with the refrigerant; and
the means for evaluating the temperature Teo comprise a temperature sensor in thermal contact with a stream of air having swept the evaporator.